1. Field of the Invention
The present invention relates to a spherical aberration corrector and a method of spherical aberration correction and, more particularly, to a spherical aberration corrector and spherical aberration correction method capable of correcting even six-fold astigmatisms by utilizing three-fold symmetric fields.
2. Description of the Related Art
In a charged particle beam instrument such as a transmission electron microscope (TEM) or scanning transmission electron microscope (STEM), aberration correction is an essential technique for obtaining high spatial resolution. Especially, positive spherical aberration produced by an objective lens that is an axisymmetric lens is a main factor limiting improvement of spatial resolution.
Today, it is widely known that this positive spherical aberration can be corrected by the use of negative spherical aberration produced by hexpole elements A. V. Crewe and D. Kopf, Optik, Vol. 55 (1980), pp. 1-10 and H. Rose, Optik, Vol. 85 (1990), pp. 19-24. H. Rose, Optik, Vol. 85 (1990), pp. 19-24 propose a spherical aberration corrector using such hexapole elements. The corrector disclosed in this non-patent document is equipped with two hexapole elements.
Referring to FIG. 5, a spherical aberration corrector disclosed in H. Rose, Optik, Vol. 85 (1990), pp. 19-24 is indicated by numeral 100 and equipped in a transmission electron microscope having an objective lens 101. The corrector 100 is mounted ahead of the objective lens 101 (i.e., mounted in the illumination system) as shown in FIG. 5 or behind the objective lens 101 (i.e., mounted in the projection system). This corrector 100 has hexapole elements 102 and 103 located on the optical axis 120, a first pair of transfer lenses 104 and 105 mounted between the hexapole elements 102 and 103, and a second pair of transfer lenses 104 and 105 mounted between the hexapole element 103 and the objective lens 101. The two stages of hexapole elements 102 and 103 are so arranged that they are coincident as viewed along the optical axis 120. That is, three-fold symmetric fields created respectively by the hexapole elements 102 and 103 bear no rotational relationship to each other about the optical axis 120. Each pair of transfer lenses 104 and 105 is made of two axisymmetric lenses. The transfer lenses 104 and 105 located between the hexapole elements 102 and 103 form, in the position of the other hexapole element 103, an image conjugate with an image formed by the hexapole element 102. The transfer lenses 104 and 105 located between the hexapole element 103 and the objective lens 101 form, in a coma-free plane 101a of the objective lens 101, an image conjugate with an image formed by the hexapole element 103. An electron beam 121 incident on the coma-free plane 101a is focused on a surface 101b of a sample.
The two stages of hexapole elements 102 and 103 produce negative spherical aberrations in mutually perpendicular directions with respect to the electron beam 121 and, therefore, the two stages of hexapole elements 102, 103 and the first pair of transfer lenses 104 and 105 cooperate to form isotropic negative spherical aberration with respect to the optical axis 120. The two stages of hexapole elements 102 and 103 act as a so-called concave lens. This negative spherical aberration suppresses the positive spherical aberration in the objective lens 101 that is a so-called convex lens.
It is generally known that a hexapole element produces three-fold astigmatism which is a second-order aberration. Accordingly, in the corrector 100 of H. Rose, Optik, Vol. 85 (1990), pp. 19-24, the three-fold astigmatism in the hexapole element 103 cancels the three-fold astigmatism in the hexapole element 102.
The aforementioned aberration correction technique can suppress the three-fold astigmatism but correct only up to the fourth-order aberration. This technique cannot completely correct still higher-order aberrations. In the above-described spherical aberration corrector, a six-fold astigmatism that is one fifth-order aberration appears in return for cancellation of the three-fold astigmatism produced by each three-fold symmetric field. Because this is a factor restricting the aberration correction, further improvement of spatial resolution cannot be anticipated.
In view of this problem, JP-A-2009-054565 discloses a spherical aberration corrector capable of correcting the six-fold astigmatism. In particular, three stages of multipole elements for producing 3 three-fold symmetric fields are arranged about the optical axis so as to be angularly spaced from each other by a given angle, thus suppressing the six-fold astigmatism.
H. Müller et al., Microsc., Microanal., Vol. 12 (2006), pp. 442-455 shows the results of a theoretical analysis of a spherical aberration corrector capable of correcting the six-fold astigmatism, and proposes a spherical aberration corrector having two stages of hexapole elements and pairs of transfer lenses in the same way as H. Rose, Optik, Vol. 85 (1990), pp. 19-24. According to the results of the analysis, in a case where an excitation current is applied to each hexapole element to maintain constant the third-order spherical aberration, the six-fold astigmatism is minimized when the length of each hexapole element taken along its optical axis assumes a certain value.
As described previously, the spherical aberration correctors disclosed in JP-A-2009-054565 and H. Müller et al., Microsc., Microanal., Vol. 12 (2006), pp. 442-455, suppress six-fold astigmatism using three-fold symmetric fields. However, they are based on a design concept different from that of the conventional spherical aberration corrector using two three-fold symmetric fields such as disclosed in H. Rose, Optik, Vol. 85 (1990), pp. 19-24. Therefore, these correctors are complex to adjust because there is a need to search for new set values such as excitation currents and the dimensions of the hexapole elements.